The present invention relates to a droplet ejection apparatus and its drive method, and in more detail, to a droplet ejection apparatus and its drive method in which, by applying a micro-vibration to a liquid meniscus in a nozzle in a degree in which the droplet is made not to be ejected, an increase of viscosity of the liquid in the nozzle is suppressed.
As a droplet ejection head in which the droplet is ejected from the nozzle when a volume of a channel is made changed, an ink jet recording head to record an inkjet image is well known.
The viscosity of the ink for the inkjet used for this inkjet recording head is, normally, about 2–5 cp (centipoise) in a room temperature. However, recently, accompanied with an offering of the high performance, additives are increased, and the ink with the high viscosity of 5–10 cp in the room temperature is also increased. Such a high viscosity ink is, as far as it is ejected under the normal temperature and normal humidity, when the drive voltage of the recording head is a little increased, it can be ejected, but because the viscosity is increased under the low temperature circumstance and is not smaller than 10 cp, and further, because the volatilization of the ink composition from the surface of the meniscus is fast under the low humidity circumstance, the ink viscosity of the nozzle surface is rapidly increased, and the ejection becomes very difficult.
As described above, it is very difficult to eject stably the ink whose viscosity is increased from the recording head under the low temperature circumstance and low humidity circumstance. For example, by also when the droplet ejection is interrupted for only a short period, when the ejection is re-started, it does not normally eject the ink from the first droplet, and the image quality is very lowered.
Conventionally, as one of countermeasures to suppress the increase of the ink viscosity of the nozzle surface, when the meniscus in the nozzle is applied micro-vibration in a degree in which the ink droplet is not ejected from the surface of the nozzle, a method by which the ink of the nozzle surface with increased viscosity is mixed with the ink in a channel is well known. And in the Patent Documents 1 and 2, when the micro-vibration is given to the surface of the nozzle under a condition in which the print is stopped, descriptions that the ink at the surface of the nozzle is mixed and the ink viscosity is lowered, are written.    [Patent Document 1] Tokkaihei No. 11-268264    [Patent Document 2] Tokkai No. 2000-94669
Under the low temperature and low humidity circumstance, because an increase of ink viscosity is very fast from the above reason, it is necessary that, after the micro-vibration is given to the meniscus, the ink droplet is ejected at once. Further, in order to conduct the stable ejection, it is necessary that the ink droplet is ejected after the vibration of the meniscus by the micro-vibration is turned down and under the condition that the meniscus position is settled at predetermined position, and when the ink droplet is ejected in a situation other than this, a size of the ejected ink droplet or the flying speed is varied, resulting in a cause of landing position error. Further, in order to effectively stir and mix the ink, it is necessary that the meniscus is largely vibrated, however, when the high frequency drive is conducted, the vibration of the meniscus needs to be attenuated early.
The method by which the meniscus of the ink is effectively swayed in this manner, or the residual vibration is effectively cancelled, can be explained from the acoustic theory as follows.
When a channel is expanded (or reduced), a pressure wave generated in the channel is, while the inversion of the pressure is repeated every 1 AL, gradually attenuated. In the case where a deformation of the channel is restored at the timing when the pressure is reversed after 1 AL after the channel is deformed, an initial pressure and a newly generated one are intensified together, and the meniscus of the ink is largely vibrated. Hereupon, the AL (Acoustic Length) is ½ of the acoustic resonance period of the channel. Accordingly, when the time after the channel is deformed, until it is restored, is made odd times of the AL, the meniscus can be largely vibrated. However, because the residual pressure is not cancelled, and the vibration of the meniscus remains, the ink droplet cannot be ejected at once.
On the other hand, in the case where the deformation of the channel is restored at the timing of 2 AL after, since the pressure is reversed and reversed again, the initial pressure and the newly generated pressure are cancelled out each other, the meniscus is not largely vibrated.
Accordingly, in the case where the time after the channel is deformed until the deformation is restored, is made even times of the AL, because the residual pressure is cancelled out, the vibration of the meniscus is turned down, and the nozzle becomes ready to eject the ink droplet at once, however, the meniscus can not be largely vibrated.
As can be seen from the above-description, in order to eject the high viscosity ink at the high frequency and stably under the low temperature and low humidity, it is necessary that the conflicting two conditions are attained, namely the meniscus needs to be largely vibrated so that the ink of the nozzle surface is effectively stirred, and the residual vibration generated by this vibration needs to be effectively cancelled out.
Both of the technology of the above-described Patent Documents 1 and 2 are technologies by which the micro-vibration pulse by a trapezoidal wave is applied on a nozzle in which the ink ejection is not conducted, and it is made in such a manner that the micro-vibration is given to the meniscus. Therefore, after the micro-vibration is given to the meniscus, because there is a time before the ink is ejected, the ink viscosity is increased again in that time, and particularly, under the low temperature and low humidity circumstance, there is a problem that the normal ejection becomes difficult. Further, in the trapezoidal wave, the circuit structure becomes complicated, and further, because the voltage sensibility becomes low, the necessary drive voltage is increased and the power consumption is increased. Further, when a number of times of applying of the micro-vibration pulse is not increased, the sufficient effect can not be obtained, and as a result, there is a problem that it results in a decrease of the printing speed.
Accordingly, the object of the present invention is to provide a droplet ejection apparatus and its drive method by which, when the liquid in the nozzle is effectively stirred, the improvement effect of the decapping characteristic is high even under the circumstance of the low temperature and low humidity, and even just after the micro-vibration of the meniscus, the droplet can be made to be stably ejected, and the high frequency and stable ejection can be conducted.
Herein, the decapping characteristic indicates a lowered amount of the initial flying speed by the so-called decapping phenomenon that, in the situation in which the nozzle surface is opened, the viscosity of the liquid is increased by the meniscus drying.